1. Field of the Invention
This invention relates generally to rotating anode X-ray tubes and is concerned more particularly with a rotatable X-ray target having a focal track provided with a heat absorbent coating.
2. Discussion of the Prior Art
Generally, a rotating anode X-ray tube comprises a tubular envelope having therein an electron emitting cathode disposed to beam high energy electrons onto a spaced anode target. The target may comprise an axially rotatable disc having adjacent its outer periphery an annular focal track made of an efficient X-ray emitting material, such as tungsten, for example. Thus electrons beamed from the cathode may be focused onto a focal spot area of the focal track to penetrate into the underlying material and generate X-rays which radiate therefrom and out of the tube.
Most of the electron energy incident on the focal spot area of the focal track is converted to heat energy which could become excessive and damage the surface of the focal track. Consequently, the target disc is rotated at a suitable high angular velocity, such as ten thousand revolutions per minute, for example, to move successive segments of the annular focal track rapidly through the focal spot area aligned with the electron beam. Thus, a one millimeter wide focal spot area on the focal track of a four inch diameter target disc would have successive segments of one millimeter width aligned with the electron beam for only about twenty microseconds, for example.
However, the penetration depth of the incident electrons is dependent on the kinetic energies of the incident electrons and the density of the focal track material. Consequently, when the focal track is made of relatively high density material, such as tungsten, for example, the incident electrons penetrate into only a thin layer of the focal track material adjacent the bombarded surface thereof. Thus, electrons having respective energies of about eighty thousand electron volts penetrate into tungsten material to a depth of only about five micrometers, for example.
As a result, a high quantity of heat is developed adjacent the bombarded surface of a segment in the brief time interval that the segment is aligned with the electron beam. Furthermore, during this brief time interval, the heat cannot diffuse through adjacent focal track material as rapidly as it is developed. Consequently, the surface temperature of the bombarded segment rises sharply to an undesirable high value, and may exceed the melting point of the focal track material. Therefore, the electron energy incident on the focal spot area, which generally is referred to as the instantaneous power rating of the tube, is limited by the rate at which heat is dissipated from the focal spot area of the focal track.
Thus, it is advantageous and desirable to provide a rotating anode X-ray tube with focal track means for increasing dissipation of heat from the focal spot area thereof.